Transmission spectroscopy is widely used for quantitatively measuring components of gaseous, liquid and solid substances. A transmission spectroscopy device can direct light into a sample, and determine properties of the sample based on how much light emerges from the sample, as a function of wavelength.
In some examples, a transmission spectroscopy device can base its calculations on the Beer-Lambert law, which relates the attenuation of light to properties of the material through which the light propagates. One way to express the Beer-Lambert law mathematically is A=e×b×c. Quantity A is the absorbance of the sample, which also equals log (1/transmittance of the sample). Quantity e is the molar absorptivity of a compound of interest in the sample. Quantity b is the optical path length traversed by light in the sample. Quantity c is the concentration of the compound of interest in the sample. In order to produce an accurate value for the compound concentration, c, one should have an accurate value for the optical path length traversed by light in the sample, b. The Beer-Lambert law is but one example of how a transmission spectroscopy device can perform its calculations. Other calculation techniques can also be used. For each of these calculation techniques, it is beneficial to know or measure the optical path length traversed by light in the sample as accurately as possible.